MESI
The protocol that keeps several CPU cores' caches agreeing on who is allowed to write to a cache line. The reason shared variables are expensive.
Every cache line carries one of four states. Modified means this core has changed the line and nobody else holds a copy. Exclusive means it is clean and lives only here. Shared means several cores hold the same clean copy. Invalid means the contents must not be used. Writing requires exclusivity, so the core broadcasts a request that invalidates every other copy — and that traffic is what costs. The practical consequence is false sharing: two threads writing to entirely unrelated variables that happen to sit in the same 64-byte line fight over it and can run tens of times slower. AMD extends the set with Owned, Intel with Forward.